header.php

<?php
if($this->session->role) {
  if($this->session->role === "user") {
    $this->load->view('menus/user_menu');
  } elseif($this->session->role === "admin") {
    $this->load->view('menus/admin_menu');
  }
}

header2.php

<?php
if($this->session->role) {
  $this->load->view('menus/' . $this->session->role . '_menu');
}